Excessive potential discharge device



Nov. 16, 1948. .1. R. M FARLlN 2,453,719

EXCESSIVE POTENTIAL DISCHARGE DE VIGE Filed Aug. 12, 1942 5 Sheds-Sheet 1 w 7 3 m I 8 l m m 4 /w I 1 a k I In! I l m 4$ in JOHN ROBERT Mc FARuN awa /0 o I r g ATTO E Nov. 16, 1948. J. R. MOFARLIN EXCESSIVE POTENTIAL DISCHARGE DEVICE 3 Sheets-Sheet 2 Filed Aug. 12, 1942 I INVENTOR JOHN Rosem Nc FARuN ATTORNEY Nov. 16, 19 8- J. R. M FARLIN v EXCESSIVE POTENTIAL DISCHARGE DEVICE 3 Sheets-Sheet 3 Filed Aug. 12, 1942 INVENTOR ATTO N BY Joan ROBERT Mc FARuN wag Patented Nov. 16, 1 948 UNITED STATES PATENT OFFICE EXCESSIVE POTENTIAL DISCHARGE DEVICE John Robert McFarlin, Philadelphia, Pa., assignor to Electric Service Manufacturing Company, a corporation of Pennsylvania Application August 12, 1942, Serial No. 454,594

13 Claims. .1

My invention is a protective device for discharging excessive electrical potentials from electrical circuits or equipment and for quickly suppressing arcs maintained by dynamic or follow current, if present, across gaps or spaces ionized by the high potential discharge.

Protective devices of the type to which my invention relates are utilized for discharging to ground surges caused by lightning to protect the insulation of electrical circuits and equipment from the effects of the excess voltages impressed thereon by the lightning and are commonly, and herein, referred to as lightning arresters or arresters regardless of their actual use. They are also used for limiting injurious voltages from other sources which tend to puncture, flash-over or otherwise deteriorate insulation unless an arrester is connected in shunt therewith, as, for instance, between the tank and windings of a transformer; between the primary and secondary windings of a transformer, or between the tank or either winding of a transformer and earth. Such devices are particularly useful Where the potential is too great to permit the interruption of an are by the normal de-ionization of gas which occurs at the first current zero of an alterhating current wave. They are characterized by their provision of a path for the ready discharge of high potential current but containing a gap or gaps having an aggregate Width too great to be fiashed-over by the normally applied dynamic potential and by their provision of means for interrupting a follow-current arc, if present, and restoring its original operating conditions.

My improvements are embodied in a protective discharge device comprising a non-conductive housing containin a plurality of gap electrodes, an arc confining slot or slots between electrodes and a centrally located tube containing a pressure relief passage communicating through ports in the tube with the slot or slots.

The electrodes and slots are arranged in series in alternation with one another and surround the aXially disposed relief passage which communicates in parallel with the arc slots when more than one of the latter are employed.

Preferably each arc slot is bounded laterally by concentrically curved, spaced walls, and hence has, in transverse cross section, the shape of a narrow rectangular slot bent around and spaced from an axis.

The ends of each arc slot are closed by ends of gap electrodes, and the peripheral surface of each gap electrode preferably contacts the inner surface of the housing and is shielded thereby from destructive action of arc gases. If, however, the device is so designed as to permit generation of sufficient pressure in the slots to cause bulging ofthe housing wall away from the outer surfaces of the electrodes, the latter may be protected from the arc gases by sleeving a shield thereon.

Some or all of the lateral walls of the slots and the wall sections or shields surrounding the elec trodes are preferably composed of or contain gas generating substances activated by an arc to produce de-ionizing gases. Thus the pressure and turbulence produced in air-filled constricting arc slots by are discharges therethrough may be augmented by the generation of de-ionizing gas in the slots from a gas generating substance contested by the arc and preferably forming one or more of the walls of the slots. For some uses, the gas generating substances may be wholly or partly dispensed with where the arc itself creates sufficient pressure and turbulence to facilitate its extinction at a current zero.

In a protective device containing arc gaps in series, there is enerally a tendency for disproportionately higher percentages of the total voltage to be impressed across the gaps nearest the line connection of the device. In my improved device I may utilize this characteristic of series gaps or I may adjust the voltage gradient to a uniform value per unit length of insulator or gap by shunting the gaps with resistance, which may be accomplished in a number of ways, so that voltage distribution across the series may be controlled by inter-gap resistance rather than by inter-gap and gap-ground capacitance.

The normal characteristics of the gaps may also be altered by shielding, that is, by surrounding a section of the device adjacent to the gaps either with metallic tubes, a winding of wire or by placing in proximity thereto metallic strips which, due to capacitative effects, modify the electrostatic field. Such shields may be made from either magnetically or non-magnetically permeable material and are preferably electrically connected to either the line or ground terminal of the device and insulated from the gap electrodes, although in lieu of this they may be electrically connected to either the line electrode of the first line gap or the ground electrode of the last ground gap, being insulated from all other gap electrodes.

The voltage impressed across the different aps may also be proportioned by varying the axial lengths of the respective slots between gap electrodes in inverse proportion to the percentages of the total voltage normally impressed slots.

from the respective slots so that the pressures l in such slots are the same or different from one another, and in other ways hereinafter described.

In my device, direct transmission of gas pressure from one are confining slot or space to the next is minimized or prevented by the gap electrodes and housing or sleeve surrounding the electrodes and the gas from each slot is vented in parallel into a common discharge passage so that uniform, progressive or otherwise different gas pressures in the different slots may be maintained" as desired to meet particular conditions. The width of the arc slots may be so designed as to produce optimum suppressive effects on the arcs without any necessity for providing for the escape through one slot of gas generated in other The areas of the vents between the arc slots and pressure relief passage may be so pro portioned to the areas of the slots and passage and'the volume of generated gas that the pressure in the arc slot-s may be so controlled as to produce optimum eifects on the are independently of the spacing of the Walls of the arc slots. In other words, slot widths and gas pressures, both of which exercise profound influences on arcing and arcs, may be independently provided for and so co-related with one another and with the character and location of the gas-generating sub stance as to provide maximum arc suppression without endangering the structure by destructive gas pressure in any part thereof.

The pressure relief passage to'which the arc slots are vented may discharge at one or both ends of the device through a port or ports so disposed that streamers expelled therefrom can- :not short-circuit any portion of the device or facilitate flash-over between the terminal electrodes thereof. If desired, the discharge port or ports from the pressure relief passage may be so disposed as to expel a blast, of non-ionized gas and de-ionized gas under pressure, into the path .of'an external arc to assist in the suppression of an are formed between electrodes of an arrester or between electrodes of series-arranged arresters or between electrodes of arresters connected respectively with phases of a multi-phase system on the ground side thereof.

The characteristic features and advantages of my improvements will further appear from the following description and the accompanying drawings in illustration thereof.

In the drawings, Fig/lis a longitudinal sectional view of a simple form of protective device embodying certain features of my invention; Fig. 2 is a transverse sectional view on the line 2-2 of Fig. 1; Fig. 3 is a fragmentary longitudinal sectional view of a protective device similar to that shown in Fig. 1 with a modified top closure;

Fig. 4 is a fragmentary sectional view of a protective device similar to that shown in Fig. 1 but provided with similar blast discharge'ports at the upper and lower ends thereof; Fig. 5 shows a protective device generally similar to that illustrated in Fig. I mounted in shunt between the line terminal and grounded tank of a transformer; Fig. 6 is a longitudinal sectional View, with parts broken "gap'electrodes H.

4 away, of a further developed protective device embodying features of my invention; Fig. '7 is a transverse sectional view on the line 'l-l of Fig. 6; Fig. 8 is a longitudinal sectional view of a protective device generally similar to that shown in Fig. 6 but having the internal gap electrodes encircled by shielding sleeves and sundry details modified; FigQ'Qis a longitudinal elevation, with parts broken away of a further modified protective device having an external are gap aligned with the blast discharge port; Fig. it shows an assembly of protective devices with an arc gap between them aligned with the blast discharge port from one of the devices; and Figs. 11 and 12 are fragmentary sectional views illustrating the partial shunting of arc gaps by resistors tending to equalize the voltages across arc slots.

in the embodiment of my invention illustrated in Figs. 1- to 5 of the drawings, a cylindrical tubular member i3 encloses a concentric, sectional tubular member i which is air-spaced from the inner wall If: of the member !3 and contains peripheral grooves I6 forming seats for annular The electrodes H are arranged in series with one another and project peripherally slightly beyond the peripheral, equilinear surface sections iii of the tube is which lie between the grooves it. The electrodes ll preferably make a close slip fit with the inner surface it of the tube i3 and center therein the tube if so that the concentrically curved concave wall iii of the tube i 53 and convex wall sections 18 of the tube Hi form are slots or spaces iii of substantially uniform narrow radial width. Such width is exaggerated the drawings for the sake of clearness, and in practice is generally of the order of ,6 or less Whenused' in connection-with ordinary distribution circuits. The gap electrodes ll separate the are slots !9 from one another; the slots it being arranged in series and in alternation with the ser es gap electrodes l1.

Each arc slot l9'surrounds, and communicates through one or more vent ports 28 with, an axial pressure relief passage 2i extending axially through the center of the tube l4 so that pressure, resulting from the action of arcs in the arc slots i9, may be relieved. through the ports 29; and passage 2i Without injury to the tubes it and M and without being transmitted from one to another of the arc slots lawhich may be substantially pressure=sealed from one another by the gap electrodes H.

"The tubes I3 and M may both be composed of fuse'd materiaL'such as porcelain, steatite, or Ba'kelite, and in such case the pressure results from thermal expansion of air in the slots is.

But preferably one or both tubes i3 and M are composed of or contain thermally activated generating material and hence the pressure resulting from air'expansion is augmented by the generation and expansion of non-ionized gas from the heat activated gas-generating substances of which either or-both of the tubes is and M are, wholly or partly, composed. The most convenient and satisfactory gas-"generating substance for such use is hard fibre, or horn fibre, but other suitable substances maybe used, if desired, such as'boricacid or compositions containing it. To increase its weatherproofness and durability, the tube I3 may be made of an inner lamina of such fibre surrounded by an outer lamina of resinous material, such as Bakelite or varnish.

The inner tube 54 is preferably'built up from end-abutted complementary similar tubular elements each containing a section of the axial passage 2| and four equi-distant radial passages 20 extending from the axial passage 2| to a Deripheral surface section l8. Each element has end sections 22 which are of reduced diameter, and complementary pairs of reduced end sections 22 of adjacent elements are telescoped within opposite ends of a metallic ring electrode l1 and form the groove It in which the electrode is seated. The electrodes make close slip fits with the peripheries of the ends 22 of each element and act as ferrules minimizing, or substantially preventing, any radial expansion, splitting or bursting f the tubular elements from the efiects of moisture, heat, or pressure.

Inner tubes M of various lengths may be built up by assembling the requisite numbers of standardized tubular elements and electrodes H for the construction of arresters of various ratings. These inner tubes |4 of various lengths may be housed within outer tubes |3 of complementary lengths, or, within limits, outer tubes of a given length may be used as housings for different length inner tubes by the use of conducting spacers, or of electrodes of different lengths, between the end elements of the inner tube l4 and the terminal electrodes of the protective device.

For example, in Fig. 1, the inner tube M is built up of four abutted fibre elements, the uppermost of which is spaced axially from the top of the tube l3 by the interposition of an elongated electrode 23 and bushing 24 between the upper element and a metallic screw cap 25 screwed onto the externally threaded upper end of the tube l3 to provide an external terminal electrode for the attachment of a line conductor A by means of a screw 26. The lowermost fibre element of the tube I4 is spaced from the bottom of the tube l3 by an electrode I1, bushing 21 and exteriorly threaded hollow nipple 28, which is screwed into the interiorly threaded lower end of the tube l3 to press toward one another and axially align the fibre elements of the tube I4 and the conducting members 21, I1, 23, 24 and 25. The nipple 28 has threaded thereon complementary nuts 29 and 30 between which may be gripped a ground conductor B.

It will be apparent that by varying the number and size of the elements 28, 21, I1, 23 and 24 a greater or lesser number of fibre tube elements may be housed within the tube l3 with consequent variation in the number of arc slots l9 and changes in electrical characteristics of the device. Arresters for low voltage circuits or equipment may require but one are slot and hence have an inner tube 4 comprising but a single standardized tubular element, such as described, with ring electrodes 11 on the reduced ends thereof and connected with, or forming, terminal electrodes of the device.

The conducting and arcing members 28, 21, I1, 23, 24 and 25 are preferably made from iron or ferrous alloy to minimize burning or melting and strengthen the arrester and its parts against splitting or bursting.

It is desirable for manufacturing economy that the tube M be symmetrically made up of standardized elements with resulting series are slots IQ of equal length, but, when it is desired that the total voltage impressed on the device be subdivided substantially uniformly across the successive arc slots, the length of the successive slot lengths may be progressively shortened by the use of progressively shorter elements or resistors or shields may be used as hereinafter described.

In lieu of the simple screw cap end closure of Fig. 1, I may provide an insulating end closure for the tubes and an insulated shield covering the connection between a line or a ground conductor and a terminal electrode, as illustrated in Fig. 3. In this construction, the conducting end bushing 24 has seated thereon the head of a conducting stud 3| having a shank passing through apertured Washers 32 and the apertured top of an insulating concave cap or hood 33. The cap is permanently bonded to the grooved end of the tube l3 by a suitable cement 34 which forms a fluid-tight joint between the periphery of the tube I3 and the skirt of the cap 33.

A sealing gasket 35 is pressed against the stud 3| and cap 33 by a flanged washer 36 and nut 31. The nut 31 has an upper face extending substantially normal to the axis of the stud 3| to form a bearing surface complementary to the dished apertured member 38 which is sleeved on the stud 3| and has lips projecting toward the surface of the nut 31 within the limits thereof to form a cup-like member which may be pressed into binding engagement with a line or ground conductor A by a threaded nut 39. The nut 39 contains a peripheral groove forming a seat for a split annular spring 40 engaged in an annular groove on the inner surface of a bushing 4| in the rotatable concave cap or hood 42. This cap 42 has a slotted flaring skirt to permit the passage of the conductor A, which is gripped between the members 38 and 31. The caps 33 and 42 minimize the possibility of accidental contact with live parts of the device as well as provide shields against inclement weather.

While it is generally desirable that a closure be provided for one end of the arrester, this is by no means essential, as both ends of the arrester may be left open to atmosphere and the upper end thereof provided with terminal elements 21, 28, 29 and 30 as shown in Fig. 4, similar to those used at the bottom of the arrester, as shown in Fig. 1.

Whether the arrester be open at both ends or have one end closed, it is electrically immaterial which end is connected with line and which end with ground when the are forming and suppressing elements in the arrester are electrically symmetrical, as in Figs. 1 to 4, but where provision is made for equi-potential distribution across the gaps, as hereinafter described, the installation of the arrester should be made to maintain such distribution by connecting the appropriate ends thereof with line and ground respectively The protective device need not be connected with continuous line and ground leads but may sometimes advantageously be so mounted as to provide an external arc gap auxiliary to the internal gap or gaps of the arrester itself. For example, as illustrated in Fig. 5, an external arc gap may be provided between an arrester electrode and a conducting member whose potential is to be equalized with that of a member on which the arrester is grounded. As shown in this figure, a grounded transformer tank 43 has an apertured, bent conducting bracket 44 secured by a nut 45 to a usual tank-cover clamp 46. The nipple 28 of an arrester, similar to that shown in Fig. 1, is passed through an aperture in the bracket 44 and secured in place by tightening up the nuts 29 and 38 on the nipple 28 against the bracket 44. The top ferrule 25 of the arrester has a slotted extension 25' in which a bent gap electrode 41 is adjustably secured by screws 48 so that its outer end may be positioned in desired spaced relation to the primary circuit connector and bushing cap 49 of the transformer. By suitably adjusting the bent electrode 4'! relatively to the bushing cap 491an external arc'gap is provided which will-be broken down Whenever the potential difference between the transformer primary and transformer tank becomes sufiiciently great to endanger the installation and thereby permit equalization of the potentials or tank 43 and cap 59 through the arrester.

When more complete'protectio'n of the arrester from weather is desired than'isefforded by the caps of Figs. 1 and 5, and particularly when the external tube i3 is composed of fibre orpaper, or when metallic sleeves'or wire windings encircle the external tube it to equalize gap voltages, or reinforce the external'tube 1-3, the arrester may be enclosed in a ceramic housing of porcelain, glass or other non-conducting'material, and such housing used as a mounting for external gap electrodes, as illustrated'in Figs. 6-to 10 .inclusive.

In the adaptations of my invention, illustrated in Figs. 6 to 8 inclusive, a cylindrical porcelain tube t, having a peripheral bead'fil for the en-- gagement of an encircling mounting bracket has its upper end covered by an apertured flanged cap 53 crimped over a sealing gasket M and-supporting an L-shaped gas-electrode An errester tube generally similar to the tube it, has threaded thereon a ferrule or cap 5'! pro vided with a threaded stud'fis extending through the apertured cap 53 and electrode 55 for the engagement of a nut 59 screwed thereon and drawing tightly together the parts through which it passes.

An insulator 65, having a ribbed periphery, contains a socket 6! in which the nut 59 is secured by suitable cement 62. The opposite end of the insulator contains a socket 63 in which a flanged nut 64 is secured by suitable cement 65.

A disk contact t6 rests on the flange oi the nut G l and a complementary dished contact 6'! is movable toward the contact 55, to grip a line or ground conductor, by a headed stud'li8 threaded in the nut 64.

The disk 66 lies substantially in the plane of the inturned end :69 of an arm of the gap electrode 55 and forms therewith an external arc gap in series with are gaps within the tube 56.

The provision of external gap electrodes, such as 86 and 69, is particularly useful in preventing the formation of corona-discharge between internal gap electrodes and eliminating danger of carbonization oi the material of the tubes in installations where high voltage iscontinuously applied between line and ground terminals. The external gap may be made of a width sufficient to electrically normally isolate the arrester from the line conductor under given dynamicvoltage conditions, but such external gap will break down or are over when harmful excess-voltages are impressed thereon and thereby bring the arrester into action to discharge the excess potential and to thereafter interrupt any flow of follow current which the external gap is incapable of interrupting.

In the protective device shown in Figs. 6 and '7, the elements within the tube 56 are similar to the elements within the tube l3 but the gap electrodes and elements of the sectional inner hollow tube H are larger and fewer'in number than the corresponding parts: I! and I4. The gap electrodes 10 discharge across the narrow arc slots 72 formed between theouter tube SEE-and the enlarged parts 13 of theinner tube'll in the 8 zsame manner asdescrib'ed in connection with Fig. lexcepting that the potentials across the gaps are substantially equalized by the shunt high resistances 14 connected with the cap 5i! and with the respective electrodes Ill by the screws 15.

The lower end of the tube at of Figs. 6- and '1 may be closed by an apertured disk 75, which is held-in place, and pressed against a peripheral sealing gasket ll, by an interiorly threaded nipplelfl screwed onto the projecting end of the nipple28. The nipple '58 is provided with a conductor connector comprising a stud l9, dished clamp and nut 6i and is so bent that its dis charge port 82 discharges gases at an angle to the axis of the arrester so as to emit a blast of non-ionized and de-ionized gases which may be directed between external electrodes of an adjacent arrester to aid in extinguishing follow current flowing between such external electrodes.

The elements within the tube 56 of 8- so modified as to expose to the arc streams a smaller area of metal and a larger area of gas generating material than do the corresponding elements of the'devices of Figs. 1 to '7, particularly when the pressure generated in the arc slots becomes'so great as to bulge the outer tube The sectional inner tube -83 of this modification is generally similar to the inner tube-H and the reduced end sections 64 of its elements are telescoped into abutting relation with one another within thin walled metallic sleeves 85 which extend'between theprotruding collars 86 of the tube 83 and between such collars and the end electrodes 87 and 88.

The conducting sleeves 85 are closely encircled by sleeves 89 which are shorter than the sleeves andare composed of non conducting and preferably gas generating material, such as hard fibre. Conducting metallic rings Si! are sleeved on sleeves 85 at the ends of the sleeves 89 and, inter-aha, form gap electrodes at the ends of arc slots 9! between the tubes 58 and protuberances The are slots'are vented'through ports 2!! and an axial passage 2?, as'hereinbefore described. Should the pressure in the arc slots 5!! rise sufiiciently high to bulge the tube the outer peripheries ofthe metallic sleeves B5 are protected from the corroding action of the arc gases by the sleeves 85;. The'outer surfaces of the latter liberate additional deionizing gases to help quench the arc.

By such construction the evolution of non-deionizing gas may be reduced and the generation of deionizing is augmented, since relatively small amount of metal and a relatively large amount of deionizing gas generating material'are exposed to the arc stream. The dynamic failure value of the device is thereby raised.

The bottom electrode is supported and positioned by an epertured bottom closure 92 which has an internally threaded flange Si? screwed on the bottom of the tube 56 and causing the closure 92 to compress the gasket 9 3 against the bottom of the tube 58. The encirclement of the lower end of the tube 55 by the external metal ferrule 93 provides greater resistance to bursting than does the use of internal bushings shown in Figs. 1 and 6 and where high pressures are probable, particularly at the discharge end of a tube, external ferrules should be usedrather than internal bushings.

The closure 92 has a threaded boss 95 depending therefrom and containing anangular passage 96 to vent the gases from the device in a desired direction. A nut 9! is threaded on the boss 95 to press the clamp 98 into wire holding relation to the closure 92.

To tend to equalize the potential across the arc gaps, a metallic shield 99 may be provided which projects downward from the ferrule 51 partly across the upper arc slot 9| to modify the electrostatic field thereof.

Fig. 9 shows a protective device similar to that shown in Fig. 8 but in which the electrical pdt'm' tials across the constricted arc slots are equalized or modified by making the upper cylindrical protuberance 88 and arc slot 9| longer than the lower cylindrical protuberance 86 and are slot 9|, which latter are the same as the correspondingly numbered parts of Fig. 8. In the device of Fig. 9 the blast from the central pressure release passage 21 is discharged upwardly through a passage I05 extending through the top of the protective device and partway through the insulator ID]. The passage I89 discharges through a downwardly inclined passage [32 which is aligned with an external arc gap between the gap electrodes I03 and [94 which are fixed respectively on the insulator I01 and between the insulator l! and the cap of the protective device. The bottom boss of the protective device may be made solid or may be apertured, as shown in Fig. 8.

As illustrated in Fig. 10, protective devices generally similar to those shown in Fig. 8 may be denuded of the top insulators and external gap electrodes shown in such figure and provided respectively with complementary external gap electrodes I05 and I06. Iwo or more protective devices so equipped may be mounted in series on an insulating base H31 by brackets I08 holding the protective devices and their external gap electrodes in fixed relation to one another so that the gap between the electrodes I05 and I06 is aligned with the discharge port 915 at the end of one of the arresters.

In Fig. 11 :there is shown a modified construction in which the inner sectional tube 83 has the reduced ends 84 of its elements abutted against one another within gap electrodes I09 having enlarged shoulders or median rings H0. The reduced ends of adjacent electrodes I09 have sleeved thereon the fibre tubes Ill and silicon carbide tubes 52. The silicon carbide :tubes contact and provide between the electrodes I05 shunt paths of high resistivity to dynamic current and low resistivity to l ghtning and further tend to equalize potentials across the arc slots 9| formed between the tubes Hi and the enlarged portions 85 of the tubes 83. This assembly of elements may be housed in any of the tubes l3 or 56 and used with or without external arc gaps.

In Fig. 12, there is shown a modified -constru0- tion in which the inner tube 83 has the reduced ends 84 of its elements abutted against one another within gap electrodes H3 having enlarged shoulders or median rings I M. The reduced ends of adjacent electrodes I I3 have sleeved thereon the fibre tubes H5 cooperating with the enlarged sections 86 of the tubes 83 to form arc slots 9!. These elements are enclosed within an impregnated resistance tube 16, which may be composed of fibre, silicon carbide, or other suitable material and held within a tube 56 or I3 by a set screw H1.

It will, of course, be understood that when the gaps are graded or their electrostatic fields are modified by shields or resistors connected with line or ground terminals the protective device should be so installed as to maintain the predetermined ratios of potentials across the arc slots.

My experiments indicate that, for optimum operating results with my invention, the distance between concentric walls of the arc slots and the gas pressures therein should be coordinated with one another and with the fault current of the circuit at the point at which a particular device is installed to secure minimum desirable impedance voltage drop and maximum arc suppression and dynamic failure voltage under the specific conditions applying.

Many factors enter into the actual operation of protective devices, such as ground resistances, severity and characteristics of the lightning discharges, and it may be economically unjustifiable to design and market devices specifically designed to exactly meet particular circuit conditions to which each device would be subjected at the point of its installation. But my improvements render practicable the production of inexpensive and simple arresters satisfactorily meeting various circuit conditions, for my invention makes practicable the production. of protective devices of various voltage ratings largely from standardized elements, with low impulse,

signed for use on circuits of low voltage rating may have but a single gas evolving slot but a plurality of gas evolving slots are preferable in protective devices designed for use on circuits 'hav-' ing a voltage rating higher than, say, 3 kv. By dividing the all-over length of the arc discharge into several sections, instead of concentrating it in a single arc slot, the pressures resulting from the discharge are distributed over a wider wall area, with consequent reduction of the tendency to distortion of the arc slot walls and bursting of the tubes. Hence higher surge currents may be safely discharged by a device having a given thickness of outer tube where a plurality of gas evolving arc slots are employed than would be possible if the device contained a single gas evolving slot of a length equal to the aggregate of the lengths of the multiple slots. The dynamic failure voltage of a given device, and hence its commercial rating. is dependent, amongst other factors, on the width of the arc slots. Less widening or distortion of arc slots occurs when using multiple slots than where a single slot is used equal in length to the aggregate length of the shorter slots. Hence my devices are preferably constructed with multiple gas generating arc slots so as to retain to an optimum degree high dynamic failure voltage characteristics and the capacity for discharging heavy surge currents.

The voltage ratings of arresters constructed one or more narrow arc slots of a gas-generat-- ing substance, or some or'all arc slot wall sections-of a ceramic or. nongas-generating material; or (C) by changing the gas pressurein the arc' slots by-alte ing the sizes ofthe radial passages through which the arc slots discl wrge into the pressurerelease passage, or the size oi the pressure relief passage; or by using terminal electrodes containing gas venting apertures atone or both ends of the arrester; or (d) by theuse-of external arc-gaps.

T-he formati'on of aroslots between concentric conca've and convex surfaces of outer and inner tubular members minimizes the disruptive ac tionof" generated pressures and provides a greatly increased surge carrying capacit and are suppressingability as compared with the surge carrying capacity and are suppressing ability ofarc slots of comparable area formed by axial or radial passages or byrpassages of ores cent cross-sectionlying between eccentric concave and convex surfaces of tclescoped members. The communication of. the respective arc slots inparallel withan axial pressure relief passage permits rapid diffusion. of generated gases throughout thearc slots; tends to equalizepressure-in the arc slots, andtends to cause a more.

or-less-evenly distributed-flow of gases from the arc slots into the discharge channel.

The concentricity of the inner and. outer surfaces ofthe arc'slotwalls, with consequent un-iforniity in the spacing of such Walls, tends to minimizeconcentration of arc discharges at any point-in the'perimeter of an electrode and. to diifuse the arc discharge completely around the perimeter soth'atallparts of the-arc are close to gas-generating or cooling surfaces oithe arcslots and the turbulence 'ofigeneratedor expand-- ed gases will, penetrate thoroughly throughout the arc.

Moreover the: formatioi-roi arc'slots between concentric surface of telescoped tubular membersfacilitates accurate control ofthe width of the arc-slots, The walls ofjarc'slots-should not b'e-s'o close together as to sufliciently impede discharge of' surge current that voltage ofsufii cie'nt magnitude' is built upacrossa given tubular element to cause a flash over over the entire elemcnt rather'than through the arc slot alone, nor should the surfaces of the-arc slots beso far apart as to permit free dischargeof an arcu'naffected'or little affected by the proximity of'the concentric Walls and the gas generated therefrom.

Any tendency or an arc to flash over-an entire tubular-element, instead of. passing through the adjacent arc slot, is effectively minimized in my arrester-b y telescoping reduced'ends of the inner tubular elements in hollow tubular electrodes so far that the length of the arc slot is lessthan one-half the flash-over distance between opposing electrodes eXteriorly of the arc slot and by limiting the pressure developed in theslots below that causing a flash-over through the relief passage. The proportioning of the length of'the flash over distances through the arc-slot and exteriorly of the arc slot may vary considerably, but generally it is desirable to make the flash-over distance exteriorly of any are slot from twoto threc'times the flash-over distancethrough the arc slot itself. This arrangement also insures that the flash-over distance between terminal electrodes, cxteriorly of the-outer tube, will be considerably longer than the aggregate ofthe internal flash-over dis tances tliroughthe arc slots sothat the-arc dis charge will always takeplace through thea1" 12 slots andthe iollow current will be extinguished in the manner intended.

For. instance, in, an arrester rated at 9.0 kv., such as illustrated in Fig. l, the flash-over distance between gap electrodes it through each arceslot I9 is less ban one-half of the fiash over distance between such opposing electrodes H exteriorly-of sucli'arc slot l9, and the external flashover distance between the terminal members 25 and 29. is over three times the aggregate internal flash-overdistances through the several arc slots 59'.

In'this'embocliment, the'distance between concentric .walls'oi each-arc-slot i9 is approximately only one two. hundred and fiftieth of the length of the arc slot l9 between adjacent gap electrodes ii and the circumferential length of the arc slot approximately three and one-half times the distance through the arc slot between adjacent gap electrodes. Such proportions afford a desirable extended Wall area relatively to the crosssectional area of the arc slot. but are not unduly critical and maybe varied somewhat to meet different conditions, and some or all of these relationships may be changed in arresters of similar I sting but'diiierent detailed design.

rhave found that. in-a typical embodiment of niv invention having a 9 kv. rating, a decrease n .028" to about .001 in the spacing between the concentric inner and outer surfaces of arc slots i9' resulted-in an increase in the dynamic failure voltage of the arrester from approximateiy 16,500 volts to above 28,000 volts, other i actors being-constant. Attire point of maximum dynamic failure voltage, the arc-slot pressure was apparently just under that required to bulge the lateral walls of the arc-slots or to cause a flashovenbetween electrodes through the relief passage; thegas vents and relief passage being so proportioned as to substantially maintain such pressure during arcing,

While higher values of dynamic failure voltage are asso-ciated'with' narrow arc slots, other factors b'esidesthe spacing between the concentric walls should lie-given consideration in designing an at" esterfor a'particular use. For instance, the clearance between the concentric surfaces should nct be so close that small changes in dimensions resulting from'atmospheric conditions or manufacturing tolerances will adversely affect the normal operation of the device, Again'the character and area of the gas-generating surface and the capacity of the pressure vents and discharge passage shouldbe'taken into consideration since the 1 de ionizing effects of-th'e arc-slots and gas are influencedby the pressure in. the arc slots and such pressure should not be so high as to bulge the walls and increase the cross sectional area of the slots nor so lowas to detract from the arc quench-- ingeffect of high pressures.

When the-efie'ctive areas of the pressure vents andreli'ef passage are so lar e that the internal pressure in the arc slots during arcing is below what maybe deemed an optimum or critical pressure, the dynamic failure voltage of the arrester will be decreased; whereas, if the efiective areas of the pressure vents and relief passage are so small that the internal pressure in the arc slots 'l'ds up suiiiciently to bulge the walls and increase the width Of the arc-slots, the dynamic failure voltage of the arrester will be lowered.

In the typical embodiment of my invention for on-a' 900G volt circuit, above referred to and illustrated inFig. 1, it was found that a desirable 1 dynamic ra uurevouace, considerably higher than the normal operating voltage of a circuit, was obtainable when the discharge passage 2| was ap-' proximately 5% of an inch in diameter and each of the four vent holes per element was approximately /8 of inch in diameter. Any substantial increase or decrease in the discharge passage area (other factors remaining the same) resulted in a marked decrease in dynamic failure voltage value. The determination of the optimum area of the relief passage, in a given design of device having given arc-slot widths, vent port areas and tube strength, for use with some maximum value of surge current of given wave shape which the device may be designed to discharge, is determinable by incrementally increasing the relief passage area from a datum preventing explosion (and thereby incrementally increasing the dynamic failure value) to an area above which the dynamic failure value again begins to decrease.

Having described my invention, I claim:

1. An arc interrupter comprising non-conducting means including a tubular element having end sections of smaller diameter than an intermediate section thereof, electrode rings sleeved on said reduced end sections and having peripheral surfaces projecting beyond the peripheral surface of said intermediate section, and a housing for said electrodes and element and having a wall complementary with said intermediate section of the latter and forming therewith an arc slot be-- tween said electrodes.

2. An arc interrupter comprising non-conducting means having complementary inner and outer concentric walls forming a curved arc slot, and gap electrodes positioned to form an arc in said slot, said means containing communicating pressure-discharge passages one of which extends substantially parallel with said are slot and forms a continuous passage from end to end of said means without intersecting said slot and the other of which extends through said inner wall and communicates with said are slot.

3. An arc interrupter comprising non-conducting means containing a pressure relief passage and a sleeve cooperating with said means and forming therewith arc slots arranged in series with one another and adjacent to said passage and communicating therewith in parallel, gap electrodes arranged in series with one another and with said slots, said electrodes alternating with said slots, terminal electrodes adjacent to the ends of said means with a direct flash-over distance between them greater than the aggregate of the flash-over distances between the adjacent gap electrodes, said passage having a discharge port remoter from one than from the other of the terminal electrodes, the flash-over distance between said port and the remoter terminal electrode being greater than the direct flash-over distance between the terminal electrodes.

4. An arc interrupter comprising a plurality of non-conducting tubular members, one of which is disposed within and spaced from the other, electrodes intersecting the space between said members to form are slots in series with one another. said inner member containing an axially extending pressure relief passage and ports connecting said passage with said slots in parallel, said electrodes being arranged in series along one of said members and discharging through said slots, one of said members comprising thermally activated gas generating material.

5. An arc interrupter comprising a tubular outer member and a tubular inner member, said members having complementary concentrically curved spaced walls, electrodes intersecting the space between said walls to form are confining slots arranged in series with one another, said inner member including a plurality of similar elements each having reduced end portions, said electrodes being sleeved on reduced ends of adjacent elements, each of said elements containing communicating pressure release passages one of which communicates with an arc confining space between adjacent electrodes and another of which communicates with a complementary passage of an adjoining element.

6. An excess-potential discharge apparatus comprising insulating means containing a pressure relief passage and also containing a slot, electrodes intersecting said slot to form arc confining slots arranged in series with one another and communicating with said passage in parallel, said spaced electrodes being arranged in series within said means and alternating with said are confining slots, and spaced gap electrodes in series with said first named gap electrodes and disposed without said means, said means including current-responsive gas-generating material activated upon flow of current and said passage having a port discharging transversely to the space between said external gap electrodes.

'7. An excess-potential discharge apparatus comprising a tubular member, a reenforcing ferrule encircling one end of said member and forming a terminal electrode, a series of gap electrodes within said member, a tubular element between said electrodes and forming with said first named member concentric curved walls for an arc slot between said electrodes, and a bushing screwed on the other end of said tubular member for tightening up the electrode and element assembly and forming a terminal electrode, one of said walls comprising a gas-generating substance.

8. An excess voltage discharge apparatus comprising a housing containing gap-electrodes and walls forming an arc slot through which one of said electrodes discharges to the other, one of said walls comprising a gas-generating substance activated by the discharge, gap-electrodes forming an arc gap exteriorly of said housing, and means for directing transversely to an are formed between said last named electrodes gas-generated within said housing by a discharge through said slot.

9. An excess voltage discharge apparatus comprising means having spaced Walls, gap-electrodes intersecting the space between said walls and forming a plurality of narrow slots arranged in series, one of said walls containing a gasgenerating substance, said gap-electrodes being arranged in series and discharging through said slots, said means containing a pressure discharge passage substantially parallel with said slots and with which said slots are connected in parallel, and terminal electrodes in series with said gap electrodes, said passage damping the discharge of gas from said slots and maintaining therein during arcing a super-atmospheric pressure.

10. An excess voltage discharge apparatus comprising means containing a pressure relief passage and having relatively rigid walls surrounding said passage and spaced to form a narrow arc slot, one of said walls containing a port through which said slot communicates with said passage, and gap electrodes at the ends of said are slot, said passage and vent being so pro- 1d portionedas to maintain in said are slot during arcing a pressure just below" that required to bulge one of said. Walls sufficiently to lower the voltage drop between the electrodes.

11. An excess voltage discharge apparatus comprising a tube containing a pressure relief passage and having an enlarged portion containing a port communicating with said passage, gap electrodes sleeved on said tube and spaced by the enlarged portion thereof, non-conducting shields sleeved on said gap electrodes, and a tube surrounding said shields and forming With the enlarged portion of the first named tube an arc slot between said electrodes.

1.2. An excess voltage discharge apparatus comprising a tube containing a pressure discharge passage and havingspaced enlarged portions each containing a port communicating with said passage, gap electrodes sleeved on said tube and spaced by the enlarged portions thereof, 0

shields. sleeved on said electrodes and coacting with said enlarged portions to form are slots between electrodes aforesaid, and a tubular resistor surrounding and in electrical contact with plurality of said electrodes, and a housing for said tube electrodes, shields and sleeve.

13. An excess voltage discharge apparatus comprising a tube containing a pressure discharge passage and having an enlarged portion containing a port communicating with said passage, conducting members. sleeved onsaid tube and spaced by said enlarged portion, rings en- 16 circling said tube adjacent to the enlarged portion thereof and a housing for said tube conducting, members and rings and forming with said enlarged portion an arc slot communicating with said passage through said vent.

JQHN ROBERT MCFARLIN.

REFERENCES CETEU The following references are of record in the file of this patent:

UNITED STATES PATENTS Number- Name Date 916,435 Gould Mar. 30, 1909 1,902,510 McEachron et al. Mar. 21, 1933 1,923,748 Roman Aug. 22, 1933 2,050,397 Torok Aug. 11,- 1936 2,151,559 McEachron 1 Mar. 21, 1939 2,169,110 Pittman Aug. 8, 1939 2,177,744 Pittman Oct. 31, 1939 2,192,773 McFarlin Mar. 5, 19 10 2,239,940 Stroup Apr. 29, 1941 2,247,352 Ackerrnann July 1, 1941 2,262,369 Opsahl Nov. 11, 1941 2,291,175 Stroup July 28, 1942 2,332,399 Pittman Oct. 19, 1943 2,357,887 Goldner Sept. 12, 1944 FOREIGN PATENTS Number Country Date 760,207 France Dec. 6, 1933 

